How did you come up with the idea for two sets of twins in the Sauveterre Quartet?

I had twins on the brain when I came up with the Wrong Heirs baby swap duet.

Like many people, I’m fascinated by twins. One of my favourite series as a child was the Bobbsey Twins.

I started thinking about two sets of identical twins, but I needed something that would arc through all four books. I came up with Trella’s kidnapping as a child, which turned them all inward with great defences, giving me some built-in conflicts to play with.